Initial Ingest OTel collector docs for Elastic Agent

[kilfoyle]

From Bill:

We've been asked to push up the release of the OTel collector and to make onboarding available via the UI in 8.15.0.

We'd like to start producing the initially internal docs for using the "OTel-mode" of Elastic Agent. In the beginning we just want to document basic (how to start OTel-mode, how to configure OTel, etc) usage but it'll probably expand out to several documents as we get closer to GA.

Resources
configuration: https://opentelemetry.io/docs/collector/configuration/
transforming: https://opentelemetry.io/docs/collector/transforming-telemetry/
architecture: https://opentelemetry.io/docs/collector/architecture/
deployment (agent): https://opentelemetry.io/docs/collector/deployment/agent/
Readme on using agent in otel mode: https://github.com/elastic/elastic-agent/blob/main/internal/pkg/otel/README.md?plain=1

To be visible only in the 8.15+ versions of the Fleet & Agent guide, starting at the 8.15 GA date (July 30).

Use Beta ([preview]) admonition:
This functionality is in technical preview and may be changed or removed in a future release. Elastic will apply best effort to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.

[bmorelli25]

@kilfoyle have you seen https://github.com/elastic/opentelemetry-dev/issues/242? Does this issue cover all of the asks in that issue?

[kilfoyle]

@bmorelli25 Thanks for the heads up! I wasn't aware of that issue.

• My PR for this issue so far includes the install steps for all platforms.
• For troubleshooting, I'd need someone to provide guidance on whatever troubleshooting steps we can add to the docs. Even for agent in regular, non-OTel mode we only have troubleshooting docs for specific scenarios and not much in the way of general troubleshooting procedures. It would be a great thing to add.

CC @strawgate

[strawgate]

documentation for troubleshooting onboarding flows should live in the onboarding guides, we can expand this out as desired

[kilfoyle]

Closed by #1250